The invention relates generally to power supplies, and deals more particularly with a plural output power supply in which one output drives a highly transient or noisy load and another output must provide a highly stable voltage.
A previously known DC power supply comprises a transformer having a primary winding supplied by an AC source, and a full wave rectifier and low pass filter connected between a secondary winding of the transformer and the load to provide the DC voltage.
FIG. 1 illustrates a more complicated, plural output power supply with a current mode control according to the prior art. A voltage source 10 is supplied to a primary winding 11 of a current sense transformer 12 and a series primary winding 13 of a converter transformer 14. Each of the transformers has its own core. The voltage source 10 can either be an AC source or a DC source (as shown) which is "chopped" by a switch 52 to yield an AC excitation. The transformer 14 has three secondary windings 22, 23 and 24 which are used to generate three respective DC outputs as follows. Each of the secondary windings 22-24 passes a portion of the AC source to a respective pair of rectifiers (diodes) 30,31, 34,35 and 36,37 and respective low pass filters 32, 36, and 38 to yield respective DC outputs 40, 44 and 46. Each of the low pass filters comprises a series inductor and a parallel capacitor. All three of the inductors of the low pass filters 32, 36, and 38 are wound around the same magnetic core so they are electromagnetically coupled to each other. This coupling, along with the coupling due to winding of all secondary windings 22-24 around the same core of transformer 14, tends to cause all of the DC outputs 40, 44 and 46 to vary proportionally to each other with load changes.
Sense transformer 12 includes a secondary winding 26. The output from the sense secondary winding 26 is rectified by rectifier 47 and develops a rectified voltage across a resistor 48. The voltage corresponds to the total current drawn by all loads and is fed back to a control circuit 50. Control circuit 50 compares this voltage to a reference voltage to control a duty cycle of switch 52 as follows. When the rectified voltage exceeds the reference voltage, the control circuit shuts off switch 52 and the drive to primary winding 13 until the beginning of the next cycle. The duty cycle controls the magnitudes of the DC output voltages. Thus, the feedback and control 50 directly regulate the output 46 and indirectly regulate outputs 40 and 44 through cross coupling.
While this prior art power supply is effective in providing three DC outputs with overcurrent protection, cross regulation and line rejection, the coupling may not always be desirable, particularly if one of the loads is subject to highly transient or noisy conditions and another requires a low noise output. For example, if one of the DC outputs drives a motor which inherently produces large transients, and another of the DC outputs drives a noise sensitive device such as a video or other electronic circuit, the noise from the motor transients will adversely be coupled to the noise sensitive electronic circuit. While it is possible to insert an additional "post" regulator between the noisy load and its DC output or between the stable load and its DC output, each additional regulator wastes power and space. Also, the noisy load may trigger the overload protection too frequently and deprive the electronic circuit of needed power. It is also possible to use additional stages of low pass filtering to better block some of the transients.
Accordingly, a first object of the present invention is to provide a plural output power supply which can better isolate a noisy load from a more sensitive load and has overload protection that is generally insensitive to the noisy load.
Another object of the present invention is to provide a plural output power supply of the foregoing type which can also provide two or more cross coupled outputs.
Another object of the present invention is to provide a plural output power supply of the foregoing types which can provide substantial isolation without additional regulation.